1. Field of the Invention
The present invention relates to a color image processing method.
Although the present invention will be described hereinbelow with respect to a video printer, the invention can be applied to any apparatus such as a color copying machine, color scanner, color printing apparatus, etc. if they obtain a color image in a manner such that an original picture or object is photoelectrically scanned to obtain a color component signal and this signal is subjected to a color process such as a masking or the like, as will be explained hereinafter.
The present invention also relates to a method of measuring an optical density regarding color lights.
2. Description of the Prior Art
FIG. 1 shows a conceptional diagram of a color reproducing method in such conventional apparatuses. First, the light component of an original 10 on a rotary drum 11 is separated into three colors by mirrors 13 and filters 12R, 12G, and 12B of R, G and B, thereby obtaining three primary color signals (R, G, B) by photoelectric converters 14R, 14G, and 14B.
Next, these (R, G, B) signals are subjected to arithmetic operations as shown in expressions (1) by a complementary converting section 6, and they are converted to densities (Dc, Dm, Dy) of the complementary colors (cyan, magenta, yellow) of each color. ##EQU1## (R.sub.0, G.sub.0, and B.sub.0 are values of R, G, and B which give white of the highest brightness).
Next, when the density values of the respective colors determined in this manner are reproduced on a paper, a mesh ratio of each size is controlled by a density-output converting section 7 by use of a masking process or the like so that desired densities (Dc, Dm, Dy) are obtained in the case of, e.g., printing or the like. Vc, Vm, and Vy are signals indicative of amounts of inks of cyan, magenta, and yellow. Although black may be used as necessary, in this case as well, amounts of inks of cyan, magenta, yellow, and black are eventually needed to be controlled to obtain desired densities (Dc, Dm, Dy).
Such a conventional method has a problem in that there is no regard to the relationship between the three primary color systems R, G, and B which were separated into three colors and the densities (Dc, Dm, Dy) of their complementary colors and no counterplan is taken to correct a deviation in color reproduction which is caused due to this.
For three-color separated signals R, G, and B even if the same signal is input, it will have become quite different color signals depending on a color filter which is used for three-color separation due to the spectral characteristic of the color filter.
In addition, for the densities Dc, Dm, and Dy of their complementary colors as well, if the spectral filters used in a densitometer which is used for measurement are not specified, even if the same density is measured, it will have become a quite different value depending on the densitometer which is used.
For example, it is assumed that two samples I and II of different spectral reflectivities are prepared. In this case, assuming that the spectral characteristics of the filter to red, green, and blue of the densitometer are R.sub..lambda., G.sub..lambda., and B.sub..lambda., the respective density values of each sample become as follows. ##EQU2## Where, Dc.sub.i, Dm.sub.i, and Dy.sub.i are density values magenta, and yellow of a sample (i), respectively; P.sub..lambda. is a spectral intensity distribution of illumination light; and .rho..lambda..sub.80 0 is a spectral reflectivity of, e.g., a standard white plate.
As shown in FIG. 2, even when .rho..lambda..sub.1 =.rho..lambda..sub.2, as will be understood from expressions (2), there is a possibility of EQU (Dc.sub.1, Dm.sub.1, Dy.sub.1)=(Dc.sub.2, Dm.sub.2, Dy.sub.2)
depending on the characteristics of the spectral filter of the densitometer.
On the other hand, the same color means that, with respect to three stimulus values (X.sub.1, Y.sub.1, Z.sub.1) and (X.sub.2, Y.sub.2, Z.sub.2) of the respective samples, the following expression is satisfied. EQU (X.sub.1, Y.sub.1 Z.sub.1)=(X.sub.2, Y.sub.2, Z.sub.2) (3)
From the definition of three stimulus values, (X.sub.i, Y.sub.i, Z.sub.i) (i=1, 2) are ##EQU3## In expressions (4), x.lambda., y.lambda., and z.lambda. are three spectral stimulus; .lambda. is a spectral intensity distribution; .rho..lambda..sub.i is a spectral reflectivity of the sample i; and K is a constant.
Therefore, in the case as shown in FIG. 2, it is obvious that (X.sub.1, Y.sub.1, Z.sub.1).noteq.(X.sub.2, Y.sub.2, Z.sub.2). In other words, in dependence on the characteristic of the filter which is used in the densitometer, even if the quite different colors are used, the respective color density values of the result of the measurement could be equal.
This means that the spectral characteristics of the separating color filter of the three-color separation system coincide with the spectral characteristics of the separating color filter which is used for density measurement. Unless they are coincident, some correction must be carried out to obtain good color reproduction.
The characteristics of spectral filters of various kinds of densitometers which are commercially available at present are not clearly specified. Therefore, the above-mentioned problem occurs and consequently, there is a problem such that those filters cannot be used to accurately examine color reproduction for color printing or color copying, etc.
Hitherto, in spite of the fact that such a problem is a serious problem regarding an essence of color reproduction, it has hardly been examined so far.